This invention relates to variable volume pumps and more particularly to an electronic combined flow and pressure control system for such pumps.
Both pressure and flow controls are known for variable displacement pumps, typically being provided as options selectable for the particular application. Pressure compensation is the most common form of control but flow compensation also finds widespread use when it is desired for example, to control the rate of movement of an actuator device. It is known, as well, to combine both flow and pressure compensation in a common application and this is typically accommodated by the use of both substantially independent options on a common pump. In the interests of conservation of energy it has become more common to utilize the full flexibility afforded by the variable displacement pump including the advantage of common flow and pressure control.
It has also become more commonplace to include remote capability in such control systems not only to achieve the advantage of selecting or modifying operational characteristics of the pump from a distance, but more importantly to achieve the end of providing such control continuously and responsively as a function of an electrical signal. Such arrangement provides a high degree of capability and flexibility for the system.
In the past there has been the capability for remote selection of both flow and pressure compensation levels in variable displacement pumps, but as indicated, these have involved the use of substantially independent devices and the cost has been prohibitive in many applications. These devices essentially consist of independent closed loop position control systems for setting a mechanical element, either of the pump itself or in a control portion of the pump. Each system typically would consist of an error and power amplifier electronic section, responsive to an input command signal and a feedback signal derived from a transducer coupled to the controlled element and, typically, further, a servo valve for delivering fluid to a control piston.
Thus, for example, in the variable volume vane type pump shown in the Schink et al. U.S. Pat. No. 3,549,281, a closed loop position control system capable of positioning an output rod, might be coupled to the compensator portion of the Schink pump so that the output rod engages the end of the spring in the compensator control, to adjust the bias produced by the spring. This then provides remote pressure compensation for the pump.
Simultaneously therewith, a second closed loop position control system might have a position transducer mechanically coupled to the cam ring of the pump and be responsive to a command signal to provide fluid pressure, by means of a servo valve acting upon pump output fluid, into the control piston of the pump which effects positioning of the cam ring. This then provides remote flow control for the pump and together with the pressure control described, results in a combined capability.
In such prior art systems it is apparent, with the increased complexity of a full flow and pressure combination system, that reliability becomes a consideration due strictly to the number of components involved. Of much greater concern, however, is the cost of implementing such systems. The great versatility afforded by the variable displacement pump has brought it into the forefront recently as the device which can meet the need of energy conservation. However, in order for it to find widespread use, it is important that the controls associated with it, which provide it with its versatility, not be cost prohibitive.